0 "I don't know the English word for it," or if it's not a particular thing, "I don't know how to say it in English" would do. "How to say" (or "how do you say") has the advantage of being quick and easy to say, and is the stereotypical way a foreigner announces that he is struggling to find the right word. But I agree, there are more better ways to express the dilemma.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.